A. Field of the Invention
This invention relates generally to machine vision systems and semiconductor chip wire bonding devices, and similar bonding apparatus, and particularly to a method and apparatus for performing automated optical inspection of a wire bond formed on a contact pad of a semiconductor chip. In order to perform an automated, in-process inspection of the ball bonds formed by the wire bonder, a system must first locate each bond formed on a chip before an inspection can take place. Applicants' Assignee's Co-pending United States patent applications:
Automated Optical Inspection Apparatus, filed Oct. 6, 1993, Ser. No. 08/132,532; and PA1 Automated Optical Inspection Apparatus Using Nearest Neighbor Interpolation, a continuation-in-part of the latter, filed Apr. 30, 1994, Ser. No. 08/236,215; and Method and Apparatus for Aligning Oriented Objects, a continuation-in-part of the latter, filed Jan. 3, 1995, Ser. No. 08/367,478; and PA1 Using Cone Shaped Search Models for Solder Ball Location filed Apr. 30, 1994, Ser. No. 08/236,211; describe systems for locating the nominal placement of a bond with sufficient speed and accuracy to permit an inspection of bond size and position to occur.
B. Background
Semiconductor devices, such as integrated circuit chips, are electrically connected to leads on a lead frame by a process known as wire bonding. The wire bonding operation involves placing and connecting a wire to electrically connect a pad residing on a die (semiconductor chip) to a lead in a lead frame. Once all the pads and leads on the chip and lead frame have been wire bonded, it can be packaged, often in ceramic or plastic, to form an integrated circuit device. In a typical application, a die or chip may have hundreds or thousands of pads and leads that need to be connected.
There are many types of wire bonding equipment. Some use thermal bonding, some use ultra-sonic bonding and some use a combination of both. A post-process inspection step, commonly called the third optical inspection, typically involves locating and inspecting the position and size of all bonds on the device, the wire connections and the wire heights using optical means to insure that a good connection was created by the bond. This is done after wire bonding is complete and before encapsulation of the integrated circuit.
Heretofore the third optical inspection has been accomplished only after the device is completely bonded and sent to a separate machine or operator, but most usually by a human operator using a microscope. The inspection is often done on a sampled basis. While sampling reduces the number of inspections, bad connections may be missed.
Vision systems or image processing systems (systems that capture images, digitize them and use a computer to perform image analysis) are used on wirebonding machines to align devices and guide the machine for correct bonding placement, but have heretofore not been used during the process for inspection purposes. Separate machines are available to perform inspections outside of and after the bonding process, but this requires another piece of capital equipment in the production line.
Bonds formed on the contact pad of the semiconductor chip are inspected to insure that their placement and size are within specified tolerances. In actual application in the field, almost all of these wirebond inspections are done manually. Manual inspection is very slow. Expensive parts may need to be inspected 100% on an individual basis, rather than sampled, and if this is done manually as well, this can further reduce productivity and add to costs.
While proprietary machines sold by Canon and Shinkawa are known to be available to inspect wire bonds, these are believed to operate only as post bond inspection systems. As a result, defective bonds may not be detected until later in the assembly process or after its completion. Productivity is thus lowered, and some defective bonds may not be detected at all.
Even when the location of a ball bond can be found quickly, however, one of the major problems both with in process or post process systems with trying to locate and measure ball bond edges in an image is the large number of confusing edges present. These are due to the circuitry, probe marks and bond pads in the image. Most semiconductor chips have a considerable amount of visual detail (such as the images of the circuits themselves) which must be circumvented in analyzing the post-bond image to find the bond edges. Some of this detail can be mistaken for parts of the bonds. Visual imperfections on the pads caused by probe marks, discoloration, or imperfect illumination further complicate these difficulties.
Existing techniques for locating circular or elliptical edges such as those formed by ball bonds on circuit pads, while they may be accurate, are often too slow to permit their use in an in-process inspection system.